1. Field of the Invention
The present invention relates to fiber-optic Sagnac interferometers. More particularly, the invention pertains to a Sagnac interferometer and a method for controlling a Sagnac interferometer.
2. Description of the Prior Art
Fiber-optic Sagnac interferometers are routinely employed in aircraft and water craft. Their use in land craft is also possible. Such devices offer autonomous determination of position in space, allowing accurate vehicle position determination when the speed is known without requiring external data (e.g. a GPS signal or satellite communication).
Fiber-optic Sagnac interferometers are generally integral parts of inertial navigation systems for determining the position or rotary motion of objects in space. Such determination is based on the association of rotation rates and accelerations acting on an object with reference to the three spatial axes of the inertial navigation system. With knowledge of rotation rates and accelerations that act on an object for a certain period of time, accurate determination of the position of the object relative to initial position is possible. When initial position is precisely known, the current absolute position of the object in space can also be determined.
Inertial navigation systems for detection of rotation rates and accelerations acting on an object for position determination are versatile. Optical, rather than mechanical effects can be employed. Such an optical system can be based on at least one fiber-optic Sagnac interferometer that employs the Sagnac effect, in which a phase difference occurs in response to a rotation between two light waves that counterrotate in a fiber-optic light guide loop. When observing the two countercirculating light waves upon emergence from the fiber-optic loop, an intensity change becomes visible that can be described by an interferometer characteristic curve. The curve defines the intensity change as a function of the phase difference between the two light waves.
A Sagnac interferometer is preferably provided for rotation of an object about each of the three spatial axes, so that a corresponding inertial system includes three Sagnac interferometers, the rotation-sensitive axes of which are orthogonal to one another. Rotation about an axis that does not coincide with one of the spatial axes, can, of course be, detected via corresponding components measured at two or three Sagnac interferometers.
The phase shift in a fiber-optic Sagnac interferometer is directly proportional to rotation speed, the length of the light path in the fiber-optic light guide loop or coil, and the diameter of the light path. Phase shift is also inversely proportional to the wavelength of the light employed.
The interferometer characteristic curve, mentioned above, that describes the dependency of the light intensity (which serves as observation variable for determining rotation) on phase difference is cosine-shaped. As a corresponding transfer function at the maximum of the cosine curve is insensitive to small inputs and the algebraic sign of the phase shift, that corresponds to the direction of rotation cannot be determined, an operating point, of the Sagnac interferometer is often shifted to lie at the point of maximum gradient of the cosine function. Sine or square-wave modulations, for example, are suitable for this purpose. Maximum sensitivity of the interferometer is thereby obtained in response to a small rotary motion.
Noise due to limited quantization ability, also called quantization noise, results from processing by the digital/analog converter. Such quantization noise can interfere with accurate rotation rate measurement.
A fiber-optic Sagnac interferometer, in which signals of an analog/digital converter are divided into MSB (most significant bit) or LSB (least significant bit) parts, is taught in published United States patent application 2010/0026535 A1 of Gregg Keith entitled “Segmented Optics Circuit Drive For Closed Loop Fiber Optic Sensors”. Parallel digital/analog converters convert 23 bits of divided MSB/LSB data into analog signals, that are combined to control an integrated optical chip. An amplifier circuit outputs MSB and LSB signals that are output to an amplifier, where they are combined into an analog signal having 23 bit precision.